Conventionally, heating elements of the type referred to above are built up on a flexible base material, such as a nonwoven material, which is attached to the body of the steering wheel by gluing, for example. A soft base material is required, especially when the covering over the base material has a special structure, such as burls or knobs or the like. This construction is selected for various reasons. First, it protects the heater on the hard body by embedding it in a soft material; second, it provides a pleasant feel in conjunction with the structures of the steering wheel.
It is, however, a problem to manufacture such a construction. To begin with, the soft base material must be fastened to the hard body of the steering wheel. In so doing, it is necessary to ensure that the heat conductor material reaches the correct place and can be fixed there without damaging the heat conductor. The covering must then be fixed carefully, so that the heat conductors do not slip to one side and are not damaged by mechanical stresses exerted on the composite of materials.
Furthermore, the finished heating element tends to suffer increased wear due to corrosion in conjunction with mechanical stresses. Corrosion is promoted due to aggressive, gaseous, liquid or dissolved solid chemicals, which are present in perspiration and detergents, or arise due to the action of heat on the several plastics that are combined by adhesives and/or solvents, and which penetrate through the covering material.
The corrosion phenomena, which occur under the conditions given above, are intensified from the start because, especially during colder times, appreciable currents flow through the heating elements. At the heating conductors that are weakened by corrosion, these currents bring about a higher than desirable voltage drop, so that temperature differences may occur which have a disadvantageous effect in the regions which are handled and, in addition, accelerate the incipient decomposition and/or dissolving process at the heating wires.
It is an object of the present invention to avoid the disadvantages that have been described and to develop a three-dimensionally deformable heating element, especially for heatable operator surfaces and for steering wheels, the deformable heating element being easily produced and being durable for a long time even under corrosive conditions. Due to the use of stranded material, it is possible to develop an even tighter and nevertheless more uniform distribution of heat on the heating element; moreover, by means of three-dimensional deformation, regions with no heating conductors and regions with a tighter arrangement of heating conductors are created, while the total wall thickness of the steering wheel heater is uniform, in order to make a concerted increase or decrease in temperature possible.
The corrosion resistance of heating conductors which are disposed on a film (base material) and over which at least one covering film is placed, can be improved significantly. This is the case especially for foamed steering wheels, for which a polyurethane foam material is pressed under pressure beneath the steering wheel cover. It has been discovered that the penetration of corrosive materials through the porous foam material, especially at places where the thickness of the film is slight, can be prevented with certainty by the use of a covering film which functions as a barrier layer for gaseous, liquid and or dissolved solid chemicals.
Moreover, if a steering wheel heater is produced with a covering film of a polyurethane material, it is significantly easier to press the polyurethane foam into the gap between the steering wheel cover and the covering material because the flow behavior of the polyurethane foam on the similar polyurethane covering layer is particularly advantageous because of the low frictional losses and the advantageous interfacial tension even when the cross sections of the gaps between the covering film and the covering material are very small.
A further advantage of the inventive heating element construction is that the device for producing three-dimensionally deformed heating elements is acted upon by smaller forces and working pressures, so that the danger of material defects is reduced. Due to the use of shrink film as a covering layer, it is possible to produce a three-dimensionally deformed heating surface in one operation with a tighter arrangement of heating conductors or with spaces free of heating conductors. This is particularly advantageous in conjunction with a self-adhering film as base material, on which the heating conductors are mounted, taking into consideration the later three-dimensional deformation. Accordingly, heating surfaces with a denser arrangement of heating conductors and with spaces free of heating conductors can be produced in one operation.